Disabling NTP was a workaround for a lack of a knob in the first place and introducing a "disableenable" type would have been more work and also not sure where to put it (system general vs. network time settings).
Just be aware of repercussions of model migration and enable/disable toggles causing model validations to fail because of cross-reference between enable settings and other (required) fields in the future... like this one https://github.com/opnsense/core/commit/54ccb747cd
PS: maybe a link to the settings page where to enable/disable would be an interesting addition to the services API?
Yes, already on Stephan's TODO list. Found out recently while testing the new WOL widget.